Among conventional pipe couplings of the type described above is a pipe coupling proposed by the present applicant in International Patent Application Publication No. 2005/028940.
The pipe coupling disclosed in International Patent Application Publication No. 2005/028940 comprises a socket and a plug that is inserted into and connected to the socket. The socket has a plurality of circumferentially spaced and radially displaceable plug-locking elements and a plurality of circumferentially spaced and radially displaceable sleeve-actuating elements provided at a position closer to a plug-receiving end opening of the socket than the plug-locking elements. The socket further has a sleeve axially displaceably provided on the outer periphery of the socket to press the plug-locking elements against an engagement groove provided on the outer peripheral surface of the plug to lock the plug as inserted into the socket.
With this pipe coupling, when the plug is not inserted in the socket, the sleeve is pressing the plug-locking elements radially inward by a cylindrical locking surface formed on the inner periphery of the sleeve. Meanwhile, the sleeve-actuating elements are in engagement with the radially inner end of a sleeve inclined surface that is inclined radially outward toward the forward end of the sleeve from the forward end edge of the locking surface of the sleeve. When the plug is inserted into the socket, first, an inclined surface at the forward end of the plug pushes the sleeve-actuating elements radially outward, causing the sleeve-actuating elements to be displaced radially outward while pressing the sleeve inclined surface, thereby displacing the sleeve axially rearward. When the sleeve-actuating elements reach the radially outer end of the sleeve inclined surface, the radially inner end of the sleeve inclined surface reaches a position where it engages with the plug-locking elements. As the plug is further inserted into the socket, the plug-locking elements are displaced radially outward while pressing the sleeve inclined surface. When the forward end inclined surface of the plug has moved forward beyond the plug-locking elements, the plug-locking elements and the sleeve-actuating elements enter a locking recess formed on the outer peripheral surface of the plug. Consequently, the sleeve is returned by a spring force to its initial position, where the locking surface of the sleeve presses the plug-locking elements in the locking recess, thereby preventing the plug from slipping out of the socket.    Patent Literature 1: International Patent Application Publication No. 2005/028940